In electrophotography, the development process of attracting charged toner particles to a latent image on a photoreceptor surface is known. In the early 1980s Canon introduced a monocomponent development system based on insulating toner, magnetic transport, and a non-contact development referred to as jumping development. The toner particles jumped across the gap between a donor roll and a photoconductive drum using large (1200 V peak-peak) AC fields superimposed across the development zone, T. Takahashi et al, Photogr. Sci. Eng. 26, 254 (1982).
Subsequently, Toshiba also developed a noncontact jumping development method described in U.S. Pat. No. 4,836,135 issued to Kohyama et al, Developing Apparatus Having One-Component Developing Agent. In addition, Toshiba developed a toner charging apparatus similar to that used in the preferred embodiment of the present invention, see Hosoya et al, IEEE-IAS Annu. Conf. Proc. (1985) p 1495.
The present invention relates to toner transport methods involving jumping or rolling of packets of toner from one electrode to another. As examples, the electrodes may be on a surface comprising a toner conveyor or on a surface comprising a writing head. The present invention thus has broad applicability to toner transport and imaging.
In an electrostatic printing device, toner particles are moved by a force qE, the product of the particle charge and the local electric field. If the electric field is too large, air breakdown will occur. The field value at which breakdown occurs is quite strongly dependent on the gap between electrodes. The maximum usable field while avoiding field emission is around 70 volts per micron when the electrode gap is around 5 microns. (See R. M. Schaffert, Electrophotography, 2nd edition, p 518 for a graph entitled Modified Paschen Curve.) To create a fast printing device it is advantageous to use the highest possible electric fields, because higher fields will accelerate the particles to higher velocities. Properly directed, higher particle velocities will translate into faster printing machines. If control electrodes are used to move the toner, then the optimum gap between the electrodes is around 5.mu.. Fortunately, modern photolithographic techniques are available to create conductive patterns approaching 5.mu. line width and space on large substrates, including rigid and flexible media.
Existing electrophotographic processes have treated toner as a continuous medium which is imaged in bulk. By contrast, the apparatus of the current invention controls and directs individual toner particles or toner packets for an extremely high degree of control over the printed image. This microscopic level of control would normally be expected to result in a very slow printing process. However, high speed is made possible and practical by the application of large scale integrated circuits (ICs) and active thin film transistor circuits (TFTs) that convert high level image information into toner controlling voltages. An example of the speed advantage in the preferred printing embodiment is that 2400 pixels are imaged in parallel to produce a new pixel line which spans the width of a page. A digital processor is provided for each pixel to be printed. As will be described, the digital processor converts 24 bit pixel words into serial bit streams that independently control the amount of toner delivered at each pixel site on the paper. Thus process complexity has been shifted from electromechanical to electronic. By virtue of the logic power and speed available in ICs and TFTs, the twin goals of high performance and low manufacturing cost can be simultaneously realized.
The preferred toner in the current invention is monocomponent, non-magnetic, and non-conducting. It has a mean diameter of around 5.mu. which matches the optimum electrode width and spacing of 5.mu., and avoids the effects of van der Waals forces which can dominate the qE force at smaller toner diameters. Preferably, the toner formulation results in resilient particles which do not fracture on collision; thus avoiding toner fragments or "fines" at very small sizes. In addition, the preferred toner shape is spherical rather than irregular; this enables rolling transport mechanisms for particles moving on a surface.
The author's co-pending applications, Ser. No. 07/658,397 filed Feb. 20, 1991 and Ser. No. 07/842,004 filed Feb. 25 1992 are incorporated herein by reference. They describe digital imaging methods based on moving packets of toner from electrode to electrode. The present invention includes rolling of spheridized toner particles to achieve toner transport with less energy applied. The present invention also includes the use of TFTs in the preferred embodiment for cost effective implementations of the writing head.